What are the four ways that statements can be in opposition?

• A. contradictory 2. contrary 3. subcontrary 4. subalternate
• B. equivalence 2. contradiction 3. subalternation 4. consistency
• C. direct 2. indirect 3. conditional 4. biconditional
• D. valid 2. invalid 3. sound 4. unsound

Answer: (B)

In this area, the focus is on how two categorical propositions relate in truth. The four classic modes of opposition are contraries, subcontraries, contradictories, and subalternation. Contraries are a pair of universal statements that cannot both be true, though they can both be false. Subcontraries are a pair of particular statements that cannot both be true, though they can both be true. Contradictories are pairs that have opposite truth values in every case—the truth of one guarantees the falsehood of the other, and vice versa. Subalternation is the link from universal to particular of the same quality: if the universal is true, the particular is true; but the truth of the particular does not, in general, guarantee the universal. Terms like equivalence or consistency describe different relations and aren’t part of this fourfold structure of opposition.